OPO mid-IR wavelength converter

ABSTRACT

A wavelength converter comprising an arsenic sulfide (As—S) chalcogenide glass fiber coupled to an optical parametric oscillator (OPO) crystal and a laser system using an OPO crystal coupled to an As—S fiber are provided. The OPO receives pump laser radiation from a pump laser and emits laser radiation at a wavelength that is longer than the pump laser radiation. The laser radiation that is emitted from the OPO is input into the As—S fiber, which in turn converts the input wavelength from the OPO to a desired wavelength, for example, a wavelength beyond about 4.4 μm. In an exemplary embodiment, the OPO comprises a periodically poled lithium niobate (PPLN) crystal. The As—S fiber can include any suitable type of optical fiber, such as a conventional core clad fiber, a photonic crystal fiber, or a microstructured fiber.

CROSS-REFERENCE

This application claims the benefit of priority based on U.S.Provisional Patent Application No. 61/101,751 filed on Oct. 10, 2008,the entirety of which is hereby incorporated by reference into thepresent application.

TECHNICAL FIELD

The present invention relates to wavelength converters for use ininfrared lasers.

BACKGROUND

Lasers transmitting in the infrared (IR) portion of the electromagneticspectrum have become very important in recent years. Of particularimportance are lasers transmitting in the mid-IR range, comprisingwavelengths between 3 and 5 μm. In this range, two atmospherictransmission bands exist, which are very useful for applications such asLight Detection and Ranging (LIDAR) systems, chemical sensing,free-space communications and IR countermeasures (IRCM). The first ofthese bands lies between about 3 and about 4.2 μm, while the second bandlies between about 4.4 and 5.2 μm. The bands are separated by anatmospheric CO₂ absorption peak which has high attenuation at about 4.3μm.

Lasers which can transmit in these two bands are limited. The majorityof such lasers are based on solid state or fiber pump lasers in thenear-IR range between 1 and 2 μm and use an Optical ParametricOscillator (OPO) crystal to convert the 1 to 2 μm pump into the longer 3to 5 μm wavelengths by a nonlinear interaction in the crystal.

Periodically Poled Lithium Niobate (PPLN) is a highly nonlinear materialwhich is very useful as an OPO for converting a near-IR laser pump tothe mid-IR. Due to the very high nonlinearity of PPLN and the ability toachieve desired phase matching by periodically poling the material,laser/OPO systems based upon PPLN have very high wavelength conversionefficiencies. PPLN based systems also can be pumped by commercialcontinuous wave (CW) lasers rather than by the high-peak-power pulsedlasers typically used with OPO materials.

Unfortunately, laser/OPO systems based upon PPLN can only produce outputhaving wavelengths up to about 4 μm due to absorption ofhigher-wavelength radiation in the PPLN crystal. Extending PPLN-basedsources to the second atmospheric transmission band beyond 4.4 μm wouldbe highly useful for the applications noted above such as LIDAR,chemical sensing, free-space communications, and IRCM, as well as manyothers.

The output from a PPLN can in turn be wavelength-converted to higherwavelengths, for example, by using an appropriate glass fiber to shiftthe wavelength the PPLN output before it is transmitted as the finaloutput of the laser.

The magnitude of the wavelength shift in the radiation emitted by theglass fiber is based on the “phonon energy” of the fiber, having unitsof cm⁻¹. The energy of the radiation, both pump and shifted wavelength,is inversely related to its wavelength, i.e.,

${E = \frac{hc}{\lambda}},$where h is Planck's constant, c is the speed of light, and λ is thewavelength of the radiation. Thus the change in wavelength λ between thepump and emitted radiation corresponds to the phonon energy of the glassand is often called the “Raman shift” or “Stokes shift,” and thethus-shifted wavelength emitted by the fiber is often called the “Stokeswavelength.”

Chalcogenide fiber Raman lasers and amplifiers can operate in the mid-IRrange. For example, U.S. Pat. No. 6,928,227 to Shaw et al., which sharesinventors in common with the present invention and is herebyincorporated by reference into the presents disclosure, describes aRaman laser and amplifier based upon arsenic selenide (As—Se) glassfiber. Raman amplification and lasers using As—Se fiber also isdescribed in P. Thielen et al., “Small core As—Se Fiber for RamanAmplification,” Optics Letters 28 [16] (2003) 1406-1408 and in S. D.Jackson et al., “Chalcogenide glass Raman Fiber Laser,” Appl. Phys.Lett., Vol. 88 (2006) 88. In the case of As—Se glass, the phonon energyof the glass is on the order of about 260 cm⁻¹. The wavelength shift inthis glass will correspond to this phonon energy. For example, for apump having a wavelength of about 1.5 μm, the wavelength shift will beabout 0.06 μm.

SUMMARY

This summary is intended to introduce, in simplified form, a selectionof concepts that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. Instead, it ismerely presented as a brief overview of the subject matter described andclaimed herein.

Features described and claimed in the present disclosure describedinvention include a wavelength converter comprising an arsenic sulfide(As—S) chalcogenide glass fiber coupled to an optical parametricoscillator (OPO) crystal and a laser system using an OPO crystal coupledto an As—S fiber. The OPO receives pump laser radiation from a pumplaser and emits laser radiation at a wavelength that is longer than thepump laser radiation. The laser radiation that is emitted from the OPOis input into the As—S fiber, which in turn converts the inputwavelength from the OPO to a desired wavelength, for example, awavelength beyond about 4.4 μm. In an exemplary embodiment, the OPOcomprises a periodically poled lithium niobate (PPLN) crystal. The As—Sfiber can include any suitable type of optical fiber, such as aconventional core clad fiber, a photonic crystal fiber, or amicrostructured fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a plot of the absorption spectrum of arsenic selenide(As—Se) fiber as a function of wavelength.

FIG. 2 is a block diagram depicting an exemplary embodiment of a lasersystem having a PPLN/OPO As—S fiber wavelength converter according tothe present invention.

FIG. 3 is a block diagram depicting aspects of an exemplary embodimentof an As—S fiber wavelength converter according to the presentinvention.

FIG. 4 depicts plots of a signal transmission spectrum for a 3.45 μmwavelength pump that has been Raman-shifted to 3.85 μm using an As—Sfiber wavelength converter according to the present invention.

FIG. 5 is a block diagram depicting aspects of another exemplaryembodiment of an As—S fiber wavelength converter according to thepresent invention.

FIG. 6 depicts plots of pump power vs. Raman laser output for varioustransmissions from an output coupling mirror for an exemplary PPLN/OPORaman laser using an As—S fiber wavelength converter according to thepresent invention.

DETAILED DESCRIPTION

The aspects and features summarized above can be embodied in variousforms. The following description shows, by way of illustration,combinations and configurations in which the aspects can be practiced.It is understood that the described aspects and/or embodiments aremerely examples. It is also understood that one skilled in the art mayutilize other aspects and/or embodiments or make structural andfunctional modifications without departing from the scope of the presentdisclosure.

For example, exemplary embodiments of a laser system described below mayinclude a continuous wave laser and an optical parametric crystal (OPO)comprising periodically poled lithium niobate (PPLN). However, it willbe noted by one skilled in the art that other forms of laser and otherOPO crystals can also be used within the scope of the present inventionso long as they emit laser radiation in the appropriate wavelengthranges in the near- and mid-infrared.

As noted above, the output from an OPO can in turn bewavelength-converted to higher wavelengths by using an appropriate glassfiber to receive the OPO output before it is transmitted as the finaloutput of the laser.

One glass that has been used in such fibers is arsenic selenide (As—Se)chalcogenide glass. For wavelength conversion from an OPO source towavelength greater than 4.4 μm, however, As—Se fiber is not viable as aRaman converter. As seen in FIG. 1, As—Se fiber exhibits very highabsorption in the wavelength range of from about 4.4 to about 5 μm dueto hydrogen selenide (H—Se) impurity absorption, making it unsuitablefor wavelength conversion in the desired range above about 4.4 μm.

In contrast, fibers comprising arsenic sulfide (As—S) chalcogenideglasses have no absorption in this wavelength range. Thus, an As—S fiberis ideal for use as a wavelength converter to achieve a final laseremission in the second band beyond about 4.4 μm.

FIGS. 2 and 3 are block diagrams depicting aspects of an exemplaryembodiment of a laser system using a PPLN and an As—S glass fiber inaccordance with the present invention. As pictured in FIG. 2, the systemcan include a continuous wave (CW) pump laser 201, a PPLN crystal 202,and an arsenic selenide (As—S) fiber 203. As described in more detailbelow, the As—S fiber can convert the PPLN output, which is in the rangeof about 2 to 4 μm, to output in the range of about 3 to 5 μm.

In some embodiments, pump laser 201 can use a neodymium-doped yttriumaluminum garnet (Nd:YAG) or crystal or an ytterbium (Yb) laser fiber asa laser material. In other embodiments, neodymium-doped yttriumorthovandanate (Nd:YVO₄) laser crystals or any other suitable rare-earthdoped solid state or fiber laser material such as ytterbium (Yb) dopedfiber, erbium (Er) doped fiber, or thulium (Tm) doped fiber can be used,and all such embodiments are within the scope of this disclosure. Notethat the laser can be either CW or pulsed. For pulsed laser, there mayor may not be a need for the fiber to be in an optical cavity formed bytwo mirrors or reflectors.

The pump laser is coupled to PPLN crystal 202 which is situated insidean oscillator formed by two or more mirrors. As described above, as alaser emission from pump laser 201 travels through PPLN 202, itswavelength increases due to the non-linear optical properties of thePPLN. For example, PPLN 202 can take an input from pump laser at a firstwavelength in the near-IR range, i.e., having a wavelength of about 1 to2 μm, and can emit at a second wavelength in the mid-IR range, withidler wavelengths between 2 and 4 μm, depending on the exact pumpwavelength, crystal temperature, and poling period of the PPLN crystal.

The idler emission of PPLN 202 is coupled to an As—S based chalcogenidefiber 203 by a coupling means (not shown). The coupling means caninclude one or more mirrors or lenses, or can include any otherappropriate means to match the mode size and divergence of the output ofthe PPLN laser to the mode size and numerical aperture (NA) of thefiber. An As—S fiber suitable for use in the present invention can beconfigured as a conventional core clad fiber, a photonic crystal fiber,a microstructured fiber, or any other kind of optical glass fibercapable of transmitting electromagnetic waves having the desiredwavelengths. For example, in the exemplary embodiment shown in FIG. 3,As—S fiber 203 comprises a conventional core clad fiber having an innercore 301 and an outer cladding 302, but as noted above fibers havingother appropriate dimensions and configurations as well as special fiberstructures such as tapered fiber structures can be used.

As shown in FIG. 3, in some embodiments, the laser cavity is formed by acavity having two mirrors 303 and 304 on the opposite ends thereof. Inalternative embodiments, the laser cavity can be formed in theAs—S-based fiber itself by mirrors deposited on the end face of thefiber or by Bragg gratings formed in the fiber. The input mirror 303will have high transmittance at the PPLN idler wavelength λ1 and highreflectivity at the first Stokes wavelength λ2 defined by the Stokesshift 204 of the fiber shown in FIG. 2 and as described above. Outputmirror 304 can have a partial transmittance at the first Stokeswavelength λ2 of the fiber and in some embodiments can have either ahigh transmittance at the PPLN idler wavelength λ1 or a partial or fullreflectivity at the PPLN idler wavelength. Thus, in some embodiments,output mirror 304 can transmit at both the input idler wavelength λ1 andthe longer Stokes wavelength λ2. Note that in some embodiments, forexample with short pulse OPO pump lasers, no mirrors 303 or 304 areneeded.

For example, if the Stokes shift of an As—S fiber is ˜350 cm⁻¹, an idleremission from PPLN 202 at a wavelength of 4 μm that is input into As—Sfiber 203 will be output by As—S fiber 203 as the final output of thelaser system after being shifted to a wavelength of ˜4.65 μm by oneStokes shift in the fiber. By tuning the wavelength of the output of thePPLN in the first atmospheric transmission band, the wavelength of thefinal output of the laser system through the As—S fiber in the secondatmospheric transmission band can be controlled.

In addition, more than one Stokes shift is possible. For example, thecavity could be resonant at the first and second Stokes wavelengths withpartial transmission at the second Stokes wavelength at the outputcoupler.

Alternatively, wavelength conversion can be accomplished by single passor double pass of the PPLN emission through As—S fiber 203. In thiscase, pump laser power, fiber core size, and fiber length can be chosento achieve the required conversion to the longer wavelength. Multipleshifts can be accomplished if desired.

In addition, in other embodiments, As—S fiber 203 can also be used as anamplifier. In such embodiments, PPLN 202 can act as a pump, and a signalin the second transmission window can be mixed with the pump in thefiber. The PPLN emission would amplify the signal in the fiber. Thesource of the signal in such an embodiment could be, for example, aquantum cascade laser diode.

Aspects of the invention will now be further described with respect tothe following Examples.

Example 1

An Acculight pulsed PPLN laser was used to pump a 1 meter length ofsmall core As—S fiber having a core diameter of 6 μm. 200 mW of emissionfrom the PPLN at an idler wavelength of 3.3 μm was launched into thefiber. The idler wavelength was Stokes-shifted to 3.85 μm and outputfrom the fiber. The Stokes-shifted radiation was monitored on the outputend of the fiber using a monochromator and InSb detector. FIG. 4 showsthe wavelength spectrum of the pump radiation 401 and the firstStokes-shifted radiation 402.

Example 2

In this example, a PPLN OPO was pumped at 1.064 μm by a Nd:YAG laser andemitted with an idler wavelength of ˜4.0 μm. The idler laser emissionwas coupled into an As—S single mode fiber 501 shown in FIG. 5, which isplaced in a cavity 502 comprising two mirrors 503 and 504 at oppositeends thereof. In this embodiment, the fiber 501 is 1 meter long, with acore diameter of 8 μm. The input mirror 503 has a high transmission at˜4.0 μm and >99% reflectivity at ˜4.6 μm. The output mirror 504 has hightransmission at ˜4.0 μm and partial transmission at ˜4.6 μm. The idlerlaser emission resonates in the cavity 502 formed by the two mirrors andthe As—S fiber and is shifted by one Stokes shift to ˜4.6 μm. Numericalaperture (NA) is 0.4, while the signal loss of the output radiation is0.1 dB/m. FIG. 6 shows the modeled laser output vs. pump and outputcoupling. Lines 601, 602, and 603 show the output power vs. input pumppower when mirror 504 has a transmission at the Stokes wavelength (˜4.6μm) of 10%, 25% and 40% respectively.

Advantages and New Features

This device greatly extends the usefulness of OPO laser systems towavelengths beyond 4 μm. Since OPO laser systems are typically verycompact and pumpable by CW lasers, Raman wavelength conversion willenable compact lasers beyond 4 μm which is not possible for existing OPOPPLN systems.

Although particular embodiments, aspects, and features have beendescribed and illustrated, it should be noted that the inventiondescribed herein is not limited to only those embodiments, aspects, andfeatures. It should be readily appreciated that modifications may bemade by persons skilled in the art, and the present applicationcontemplates any and all modifications within the spirit and scope ofthe underlying invention described and claimed herein.

1. A wavelength converter for a laser system, comprising: a solid coreclad arsenic sulfide (As—S) glass fiber operatively coupled to anoptical parametric oscillator (OPO) crystal; wherein the solid core cladAs—S glass fiber is configured to receive first laser radiation from theOPO at a first wavelength, the first wavelength being an idlerwavelength of the OPO, and to emit second laser radiation at a secondwavelength, the second wavelength being longer than the firstwavelength, a difference between the first and second wavelengthscorresponding to a Stokes shift associated with the solid core clad As—Sglass fiber.
 2. The wavelength converter according to claim 1, whereinat least one end of the As—S glass fiber has a tapered structure.
 3. Thewavelength converter according to claim 1, wherein the first and secondlaser radiation comprise radiation having a wavelength in themid-infrared range.
 4. The wavelength converter according to claim 1,wherein the first laser radiation comprises radiation having awavelength between about 2 μm and about 4 μm, and the second laserradiation comprises radiation having a wavelength between about 3 μm andabout 5 μm.
 5. The wavelength converter according to claim 1, whereinthe As—S glass fiber is situated in a laser cavity comprising an inputmirror and an output mirror, the input mirror of the laser cavity beingat a first end of the As—S glass fiber and the output mirror of thelaser cavity being at a second end of the As—S glass fiber; and furtherwherein the input mirror has a high transmittance at the firstwavelength and a high reflectance at the second wavelength and theoutput mirror has at least a partial transmittance at the secondwavelength.
 6. The wavelength converter according to claim 5, whereinthe output mirror has at least a partial transmittance of the firstwavelength so that the second laser radiation comprises both the firstand second wavelengths.
 7. The wavelength converter according to claim1, wherein the As—S glass fiber comprises a laser cavity formed by aplurality of Bragg gratings in the As—S glass fiber; and further whereinthe first laser radiation at the first wavelength is converted to thesecond laser radiation at the second wavelength due to interaction ofthe first laser radiation with the Bragg gratings in the As—S glassfiber.
 8. The wavelength converter according to claim 1, wherein the OPOcrystal comprises a periodically poled lithium niobate (PPLN) crystal.